This invention relates to novel synchronous motors and the control units thereof.
In the prior art, a DC motor is often utilized for controlling the speed of servomechanism for its simpleness in operation and excellent control. However, since a DC motor is equipped with brushes and commutators, it inconveniently requires periodic maintenance and inspections in order to keep normal operation. Since semiconductors such as power transistors along with control technology have made a remarkable progress in recent years, a demand for motors which do not need maintenance is keenly felt. Studies have been conducted on control by AC motors and some have been put into practice.
FIGS. 1 and 2 show an example of the structure and the control unit of a prior art synchronous motor of an electromagnetic field type wherein a synchronous motor 10 has armature windings 11 of three phases (U-phase, V-phase and W-phase) which are wound around a stator, and a rotor 12 is wound with a field winding 13. FIG. 2 shows such a conventional synchronous motor 10 in cross section wherein a cylindrical casing 16 houses a laminated electromagnetic iron core 15 for the stator. On the inner periphery of the electromagnetic iron core 15 are bored plural slots 14 at uniform intervals on which armature windings 11 are wound. The armature windings 11 are formed in the slots 14 by distributed windings (or concentrated windings). A salient pole rotor 12 of a bipolar type is provided in the vacant space of the electromagnetic iron core 15 of the stator in a rotatable manner. The field winding 13 around the rotor 12 is supplied with current from a field circuit outside the motor via a slip ring.
The synchronous motor 10 of this type is controlled by a control unit such as the one shown in FIG. 1. A detector 4 is mechanically connected to a rotational shaft of the rotor 12 for detecting rotational speed and position thereof. Detection signal DS from the detector 4 is inputted in a rotor position detection circuit 5 and a speed detection circuit 6 for detecting the position and the speed of the rotor 12, respectively. A speed command SI from another control unit (for instance, a computer) is inputted into a subtractor 1, and the speed deviation ES between a speed signal SD from the speed detection circuit 6 and the speed command SI is inputted into a PID (proportional, integral and difference) compensating circuit 9, and a torque command ESA output therefrom is inputted into an armature current command circuit 2 to form command currents SI.sub.u, SI.sub.v and SI.sub.w for the three-phase armature. These command currents SI.sub.u through SI.sub.w are inputted into an armature current control circuit 3 to be supplied to the three-phase armature windings 11 of the synchronous motor 10 as the armature currents I.sub.u, I.sub.v and I.sub.w. The field winding 13 of the rotor 12 is supplied with current via a slip ring from a field current control circuit 8 in correspondence with a field current command FS from a field current command circuit 7. The detailed construction of the armature current command circuit 2 is shown in FIG. 3. The command circuit 2 is provided with memories (e.g., ROMS) 22U, 22V and 22W which digitally store the sine-waves in the U-phase, V-phase and W-phase, respectively. Corresponding to the value of the rotor position signal RP from the rotor position detection circuit 5, sine-wave data stored in the memories 22U through 22W can be accessed from a memory addressing circuit 21. The sine-wave data which are accessed from the memories 22U through 22W are inputted into D/A converters 23U through 23W to be converted to analog signals, respectively. The sine-wave signals in analog are respectively inputted to multipliers 24U through 24W. The torque command ESA obtained from the PID compensation circuit 9 is inputted into the multipliers 24U, 24V and 24W and multiplied with the sine-wave signals from the D/A converters 23U, 23V and 23W, respectively. An electrical current in accordance with the result of above multiplication which indicates position and speed deviation ES is inputted into the armature current control circuit 3 as armature current commands SI.sub.u, SI.sub.v and SI.sub.w. Subsequently, current-controlled armature currents I.sub.u, I.sub.v and I.sub.w are supplied to the synchronous motor 10.
In such a structure as stated above, the armature current command circuit 2 reads out the digital values of the sine-waves stored in the memories 22U through 22W in correspondence with the rotor position signal RP from the rotor position detection circuit 5. After being converted into analog signals, they are multiplied in accordance with the speed deviations ES between the speed command SI and the actually detected speed signal SD from the speed detection circuit 6. The armature current command circuit 2 therefore outputs armature current commands SI.sub.u through SI.sub.w so as to make the speed command SI coincide with the speed of the rotor 12. In this manner, the synchronous motor 10 can control the rotation of the rotor 12 at the rate of the speed command SI through the armature current control circuit 3.
The torque T generated from the synchronous motor 10 can be expressed by the equation below if it is assumed that the position of the rotor 12 is .theta., the relative phase between the armature current I and magnetic flux density B is .alpha., the magnetic flux density B is distributed in a cosine-wave around the position .theta. of the rotor 12, and phase armature currents I.sub.u through I.sub.w are respectively distributed in cosine-wave synchronization with the magnetic flux density B which crosses perpendicularly. ##EQU1##
Wherein B.sub.o and I.sub.o respectively represent the maximum values of the magnetic flux density and the armature current when .theta.=0. If the magnetic flux density B is synchronized with respective currents of the three phases, it holds that .alpha.=0.degree. and EQU T.varies.(3/2B.sub.o I.sub.o ( 2)
If an ideal control is carried out, the output torque T of the synchronous motor 10 is relevant only to the magnetic flux density B and the magnitude of the armature current I. Therefore, if it is assumed that the magnetic flux density B is constant, since the output torque T of the motor becomes dependent only on the magnitude of the armature current I, the torque T can be controlled as excellently as in the case of the DC motor.
However, since such a conventional synchronous motor 10 is equipped with the field winding 13 on the rotor 12, it requires a power amplifier and a control circuit for controlling the field current as well as a slip ring or a rotary transformer for feeding the field current to the side of the rotor 12. Furthermore, if a permanent magnet is used on the rotor since a field of the synchronous motor, as the permanent magnet per se is expensive and requires a complex structure for fixing the permanent magnet on a shaft and so on, it presents difficulties cost-wise in making the capacity larger. Moreover, since the magnitude of the field is constant, the inductive voltage of the stator winding becomes proportional to the rate of rotation, thereby placing an upper limit on the controllable rotational speed range.